These types of sensors are usually designated as pneumotachographs, since they transform a flow signal into a pressure signal. Usually, pressure drop by obstruction of the sensor is converted into an electric signal by means of a differential pressure transducer, and may be integrated into a volume calculation, particularly to measure the breathing function in patients.
The ratio of the flow passing through the sensor to the respective pressure drop through the obstruction defines the sensitivity or gain of the sensor. Constant gain or linear ratio between the flow and pressure drop represents an ideal response, facilitating the calibration process and minimizing the need for additional calculations to obtain the correct flow.
The use of said pneumotachograph type sensors is one of the most widely spread for medical application, especially in anesthesia equipment and lung ventilators. Pneumotachographs present various advantages over other types of flow sensors, e.g.: the part applied to the patient is free from electric connections, has low cost, is compatible with cleaning and sterilization processes, immune to electromagnetic interference and low-sized.
The use in patients requires some attributes such as precision at low and high flows, relative immunity to patient secretions and enough resistance to support cleaning and sterilization processes.
These requirements have been met, to some extent, by two types of flow sensors from the class of pneumotachographs, known as fixed area sensors and variable area sensors, or even fixed orifice sensors or variable orifice sensors.
Fixed area sensors are disclosed by the U.S. Pat. Nos. 4,047,521; 5,088,332; 5,379,650; and 6,312,389.
U.S. Pat. No. 4,047,521 discloses a flow sensor comprising a tube inside which two cylindrical bodies, aligned and in opposite senses, are radially projected, constituting an obstruction element. This system aims to be relatively simple and easy for manufacture in comparison with Fleisch type pneumotachographs.
Fleisch type pneumotachographs have been extensively used through the years, mainly in lung tests and research applications, due to their linear response, i.e. the pressure drop by means of the obstruction of sensor is proportional to the flow, dismissing the need for corrections by software. However, these sensors make use of a screen as an obstruction element, liable to the accumulation of patient secretions, making this kind of sensor inadequate to continuously monitor the patient. Furthermore, they are relatively large and heavy. Despite attempting to solve the problems of the Fleisch pneumotachograph, the sensor as disclosed by the U.S. Pat. No. 4,047,521 shows non-linear response, since the pressure drop is a square function of flow, thus resulting in low sensitivity at low flows and high resistance at higher flows.
U.S. Pat. No. 5,088,332 discloses another alternative, wherein the fixed obstruction is constituted by wings laterally located inside the flow passage tube, aimed to minimize the accumulation of liquids and secretions in comparison with other sensors, such as Fleisch type. Also in this configuration, the sensitivity to low flows constitutes the main problem. Furthermore, geometry as proposed does not avoid the accumulation of secretions along the obstruction. Wing intersection with the internal wall of the tube, where flow passes, creates a lengthwise region with low speed and high friction, favoring the accumulation of secretions during continued use.
U.S. Pat. No. 5,379,650 discloses a sensor with similar objects to patents as previously disclosed. That patent discloses a sensor which obstruction is located lengthwise and extends diametrically between the walls of the tube, being pressure measurement points located axially at the ends of the obstruction.
The obstruction intersection with tube walls results in lengthwise extension, which, as in the case of the patent as previously mentioned, facilitates adherence and secretion accumulation. That sensor also presents quadratic relationship between pressure and flow, thus resulting in low sensitivity under low flow.
As previously stated, another alternative for obstruction sensors are orifice or variable area sensors, as disclosed by the U.S. Pat. Nos. 4,006,634, 4,083,245, 4,688,433, 4,993,269 and 5,038,621.
The advantages of variable area sensors are related to the level of sensitivity in low flow measurements and the reduction of risks of secretion accumulation.
U.S. Pat. No. 4,006,634 discloses a sensor wherein the obstruction comprises a set of flexible radial flaps which are crosswise located and flexed according to the passage of flow, thus proportionally increasing the area of passage. According to said patent, the appropriate sizing of that arrangement may result in linear response. This alternative requires extreme care for handling, cleaning and sterilization, due to the fragile flexible flaps, usually constructed from very thin metal plates. The presence of secretion and condensate accumulation may affect the response by this model of sensor.
Another problem in the sensor of the state of the art relates to vibration, especially due to the presence of contaminants and higher flows.
U.S. Pat. No. 4,083,245 discloses a sensor with simpler embodiment than multiple flaps, just comprising one elastic flap, to increase the passage of one orifice through flow passage. However, the precise alignment of the flap over the orifice is essential to keep the pressure under low flows, a condition which is hard to maintain in the presence of secretions or condensation.
U.S. Pat. No. 4,688,433 discloses a sensor wherein the obstruction is constituted by a rigid member radially located to the sense of flow, having a second flexible member assembled upstream. For low flows, the flexible member moves freely and the sensor behaves as having variable area. For higher flows, the excursion (movement) of the flexible member is limited by the rigid member and the sensor now shows the characteristic of a fixed area sensor, resulting in a sigma curve. In this case, there is a transition point characterized by limiting the movement of the flexible member. Furthermore, as in previous cases, the position of the flexible member in the region of nearly zero flow is critical, depending on the rest position of the flexible element, susceptible to contamination and misalignment due to continued use.
U.S. Pat. No. 4,993,269 discloses a sensor wherein the obstruction comprises a flexible element away from the internal walls of the sensor tube, with the purpose to avoid the accumulation of secretions. In this case, for higher flows, the pressure drops more than proportionally, thus reducing sensor sensitivity in higher flows, besides the susceptibility to vibration due to the free flexible element.
U.S. Pat. No. 5,038,621 discloses a variable area sensor wherein the obstruction comprises an elastic membrane including three flaps cut on the membrane itself, opening with the passage of flow. The obstruction as disclosed by that patent aims to obtain higher sensitivity under low flows. However, the opening of side flaps with no anchoring reduces the natural frequency of the system and provides the occurrence of vibrations at higher flows or under continued flow conditions.
As explained, the need for improvements in sensors of the state of the art concerning sensitivity, stability, immunity to secretions and resistance to support the required cleaning and sterilization processes is evident.